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Homework Help: Work done during collision?

  1. Aug 1, 2015 #1
    1. The problem statement, all variables and given/known data
    Two blocks have masses m1 and m2 and speeds v1 and v2. The objects slide directly toward each other along a frictionless horizontal surface and collide. After the collision, block m1 is at rest. Find the work done by the contact force on block m2 during the collision.

    2. Relevant equations
    W = 1/2*m*v_f^2 - 1/2*m*v_i^2

    m*v_1 + m*v_2 = m*v_1' + m*v_2'

    3. The attempt at a solution
    m_1*v_1+m_2*v_2 = 0+m_2v_2'

    v_2' = (m_1*v_1+m_2*v_2)/m_2

    W = 1/2*m_2*((m_1*v_1+m_2*v_2)/m_2)^2 - 1/2*m_2*v_2^2

    What am I doing wrong?
  2. jcsd
  3. Aug 1, 2015 #2
    Post edited .
    Last edited: Aug 2, 2015
  4. Aug 2, 2015 #3
    m_1*v_1+m_2*v_2 = 0+m_2v_2' is wrong all v-s are speeds to be corrected as
    m_1*v_1- m_2*v_2 = 0+m_2v_2' ----------------------------- (1), hence
    v_2' = (m_1*v_1-m_2*v_2)/m_2 or
    W(by m1 on m2) = 1/2*m_2*((m_1*v_1-m_2*v_2)/m_2)^2 - 1/2*m_2*v_2^2, simplify to get the correct answer.

    Work done on mass m1 by mass m2 = -1/2*m_1*v_1^2
  5. Aug 2, 2015 #4
    ahhh yes, thank you!
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